Tafenoquine succinate inhibits the growth of the equine piroplasmosis hemoparasites Theileria equi and Babesia caballi.
Abstract: Equine piroplasmosis (EP) is a tick-borne disease of equids caused by the intraerythrocytic apicomplexan parasites Theileria equi, Babesia caballi and the recently identified Theileria haneyi. Acute cases can be severe, with anemia, jaundice, abortion or sudden death. Survivors remain lifelong carriers, serving as reservoirs for tick-borne and iatrogenic transmission. No vaccines are currently available, and control strategies rely heavily on accurate diagnostics and chemotherapeutic intervention. Imidocarb dipropionate (ID) is the current standard of care for both acute treatment and radical cure. However, growing concerns regarding ID-resistant parasite strains and its associated toxicity have highlighted the urgent need for novel, safer and more effective antiparasitic agents. Here, we assessed the in vitro efficacy of tafenoquine succinate (TFQ), a synthetic 8-aminoquinoline with broad antiparasitic activity, against T. equi and B. caballi as a potential treatment for equine piroplasmosis. Methods: The effect of TFQ on T. equi and B. caballi was evaluated in vitro in parasite cultures. The percentage of parasitized erythrocytes was measured by flow cytometry, and the effect of TFQ on parasite growth was compared to that of ID. TFQ toxicity on horse peripheral blood mononuclear cells (PBMCs) was assessed via a colorimetric metabolic assay. Results: TFQ reduced T. equi parasitemia in a dose-dependent manner, matching ID efficacy at 72 h. For B. caballi, TFQ had no effect at 5-10 µM but inhibited growth at 15 µM, similar to the results obtained with ID. TFQ exhibited approximately threefold greater potency against T. equi [half-maximal inhibitory concentration [IC] 5.90 μM, 95% confidence interval (CI) 4.99-5.96; 99% inhibitory concentration (IC) 60.74 μM, 95% CI 37.41-113.3] compared to B. caballi [IC 14.5 μM, 95% CI 13.81-15.23; IC 20.44 μM, 95% CI 17.77-28.84]. The narrower confidence intervals for T. equi suggest a more consistent antiparasitic response across replicates. Cytotoxicity assays showed no toxic effects on equine PBMCs at 2.5-5 μM (P > 0.05), while concentrations ≥ 10 μM indicated potential toxicity. These findings suggest that TFQ selectively targets parasites over host cells, supporting its therapeutic potential. Conclusions: TFQ significantly inhibited T. equi and B. caballi growth at doses tolerated by equine PBMCs, supporting its potential as an alternative treatment for EP and warranting further in vivo study.
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Publication Date: 2026-01-27 PubMed ID: 41593688PubMed Central: PMC12862906DOI: 10.1186/s13071-026-07262-yGoogle Scholar: Lookup
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Summary
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Overview
- This study investigates the effect of tafenoquine succinate (TFQ), a synthetic antiparasitic drug, on the growth of equine piroplasmosis parasites Theileria equi and Babesia caballi in vitro, aiming to explore TFQ as a potential alternative treatment for equine piroplasmosis (EP).
Background
- Equine piroplasmosis (EP) is a tick-borne disease affecting horses and other equids caused by intraerythrocytic parasites: Theileria equi, Babesia caballi, and Theileria haneyi.
- Acute infection symptoms can be severe, including anemia, jaundice, abortion, or sudden death.
- Infected animals often remain lifelong carriers, posing a risk for continued parasite transmission through ticks or medical procedures.
- Current control strategies lack vaccines and mostly rely on diagnosis and chemotherapeutic treatment.
- Imidocarb dipropionate (ID) is the standard drug for both treatment and cure but has issues such as emerging parasite resistance and toxicity.
- There is an urgent need for new, safer, and more effective treatment options.
Objective
- The study aimed to evaluate in vitro the efficacy of tafenoquine succinate (TFQ), an 8-aminoquinoline compound with broad antiparasitic properties, against T. equi and B. caballi.
- Also, it assessed the toxicity of TFQ on horse peripheral blood mononuclear cells (PBMCs) to gauge safety.
Methods
- Parasite cultures of T. equi and B. caballi were treated with varying concentrations of TFQ.
- Parasitemia (percentage of parasitized red blood cells) was measured using flow cytometry at different time points, especially at 72 hours.
- TFQ’s antiparasitic effect was compared with the current standard drug, imidocarb dipropionate (ID).
- Toxicity of TFQ on horse PBMCs was evaluated via a colorimetric metabolic assay, assessing cell viability at various TFQ concentrations.
Results
- TFQ inhibited T. equi growth in a dose-dependent manner, achieving efficacy comparable to ID at 72 hours.
- For B. caballi, TFQ had no inhibitory effect at lower doses (5-10 μM) but effectively inhibited growth at 15 μM, matching ID’s effects.
- TFQ demonstrated about threefold greater potency against T. equi compared to B. caballi:
- T. equi IC50: 5.90 μM (95% CI 4.99-5.96), IC99: 60.74 μM (95% CI 37.41-113.3)
- B. caballi IC50: 14.5 μM (95% CI 13.81-15.23), IC99: 20.44 μM (95% CI 17.77-28.84)
- Confidence intervals for T. equi were narrower, indicating more consistent responses in experiments.
- Toxicity testing showed no harmful effects on equine PBMCs at TFQ concentrations between 2.5-5 μM (P > 0.05), but concentrations ≥10 μM suggested potential toxicity.
- This selective toxicity indicates that TFQ inhibits parasites more than host cells at effective doses.
Conclusions and Implications
- TFQ effectively inhibits the growth of EP-causing parasites at concentrations tolerated by horse immune cells in vitro.
- These findings position TFQ as a promising alternative or adjunct to current therapies for equine piroplasmosis.
- Due to concerns regarding ID resistance and toxicity, TFQ’s potential safety and efficacy merits further investigation.
- Future studies should focus on in vivo validation of TFQ’s therapeutic potential in infected equids.
Cite This Article
APA
Cardillo NN, Villarino NF, Kappmeyer LS, Chung CJ, Suarez CE, Bastos RG.
(2026).
Tafenoquine succinate inhibits the growth of the equine piroplasmosis hemoparasites Theileria equi and Babesia caballi.
Parasit Vectors, 19(1), 61.
https://doi.org/10.1186/s13071-026-07262-y Publication
Researcher Affiliations
- Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, WA, USA.
- Animal Disease Research Unit, USDA-ARS, Pullman, WA, USA.
- Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, Pullman, USA.
- Animal Disease Research Unit, USDA-ARS, Pullman, WA, USA.
- Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, WA, USA.
- Animal Disease Research Unit, USDA-ARS, Pullman, WA, USA.
- Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, WA, USA.
- Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, WA, USA. reginaldo.bastos@usda.gov.
- Animal Disease Research Unit, USDA-ARS, Pullman, WA, USA. reginaldo.bastos@usda.gov.
MeSH Terms
- Animals
- Babesia / drug effects
- Babesia / growth & development
- Horses
- Theileria / drug effects
- Theileria / growth & development
- Horse Diseases / parasitology
- Horse Diseases / drug therapy
- Babesiosis / parasitology
- Babesiosis / drug therapy
- Theileriasis / drug therapy
- Theileriasis / parasitology
- Aminoquinolines / pharmacology
- Antiprotozoal Agents / pharmacology
- Leukocytes, Mononuclear / drug effects
- Leukocytes, Mononuclear / parasitology
Grant Funding
- # 2090-32000-044-000-D / USDA-ARS CRIS
Conflict of Interest Statement
Declarations. Ethics approval and consent to participate: Blood from healthy, hemoparasite-free horses were used in this study to maintain in vitro cultures of T. equi and B. caballi. Blood collections were performed according to protocols approved by the Institutional Animal Care and Use Committees of the University of Idaho (protocol # 2024–26) and Washington State University (protocol # 6982). Consent for publication: Not applicable. No personal data or images of individuals were included in this study. Competing interests: The authors declare no competing interests.
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